Awareness of the risks and prevalence of UV-induced skin cancer and consciousness of the damaging cosmetic effects of ultraviolet radiation have fostered recognition in recent years of the desirability of avoiding or controlling the physiological damage wrought by harmful ultraviolet radiation, especially solar ultraviolet. UVC radiation is generally not problematic as little if any penetrates the atmosphere, but may be hazardous if artificial sources such as germicidal lamps or high or low pressure mercury arc lamps are employed.
In contrast to the UVC waveband which is filtered out, both UVA and UVB radiation reach the earth's surface from the sun and may be harmful. Longer wavelength ultraviolet rays in the UVA region, adjacent the visible spectrum, which reportedly constitute 99% of the UV energy reaching the earth's surface, are considered to be the primary cause of tanning but to cause little burning. However, in the longer term UVA rays are also believed responsible for skin aging, causing blotching, freckling, wrinkling and comparable problems. Ultraviolet radiation in the shorter wavelength UVB region of from 290 to 320 nm, though comprising only about 1% of the UV energy is considered more significant in causing burning. Accordingly, protection against UVB radiation has traditionally been a primary target of sunscreen formulation. More recently, there has been a recognition of the desirability of also protecting against UVA radiation to reduce long term skin deterioration.
More seriously, both the UVA and UVB wavebands may be implicated in skin cancers including dangerous melanomas. Skin cancer is by far the most prevalent of all cancers and the incidence in the US is increasing rapidly. For these and other reasons, consistent use of a topical sunscreen is now strongly recommended and is widespread. In light of this need, the art contains many proposals for sunscreen agents and compositions intended to provide protection from damaging ultraviolet radiation.
A wide variety of UV-protective agents suitable for topical application in creams, sprays, lotions and the like is known and has been used in sunscreens. However, most such agents are organic chemicals that are prone to photodegradation and may cross-react with other components of desirable topical formulations. Furthermore, being absorbable into the skin, organic sunscreens may irritate the skin or cause other dermatological or allergenic problems. The art also contains proposals for broad spectrum UV protection.
For example, Deckner U.S. Pat. No. 5,783,174 describes sunscreen compositions intended to provide broad protection in both the UVA and UVB regions which compositions combine a UVA-absorbing dibenzoylmethane sunscreen with a UVB-absorbing benzylidene camphor sunscreen. This composition is intended to avoid problems of photochemical instability attributable, according to Deckner, to photoinduced interaction between dibenzoylmethane and the widely used UVB absorber octylmethoxy cinnamate.
A more recent demand for both higher SPF (“sun protection factor”) values and for broad-spectrum protection, has led manufacturers to combine several different organic chemical ingredients, aggravating potential problems of photo-induced cross reactivity. To overcome this problem, Lapidot, et al. describe in U.S. Pat. No. 6,436,375 a method for microencapsulating at least one of two or more active sunscreen ingredients which are photo-unstable when formulated together. The active ingredients can be selected for UVA and UVB activity and can be encapsulated in separate sol-gel microcapsules. A drawback of Lapidot et al.'s proposal is that even if problems of cross-reactivity and photodegradation can be overcome, which may or may not be the case, Lapidot et al.'s method still requires use of active organic sunscreen agents which may be harmful or irritating when absorbed. Furthermore, use of sol-gel microcapsules adds complexity and expense and may be undesirable in some topical formulations. Such microcapsules may also raise issues of dispersibility and compatibility with other ingredients of end product formulations.
There is accordingly a need for a UV-protective agent having broad spectrum activity against harmful solar radiation and which does not depend upon organic chemicals. In light of the problems with organic materials, the suitability of inorganic materials may be considered. Several commercially useful inorganic UV-protective agents are known, notably titanium dioxide, zinc oxide and iron oxide. Iron oxides, however are usually colored or black and therefore have rather limited use in sunscreen applications. Also, they may not be approved for use as sunscreens by regulatory authorities such as the US FDA.
Thus, considerable difficulties face a formulator or other worker seeking broad spectrum UV-protective agents that will be satisfactory for widespread commercial use in a full range of topical commercial products.
Elsom et al. (WO 90/11067) provide single-species metal oxide sunscreen compositions. Specifically provided are sunscreen compositions containing blends of titanium dioxide powders having a particle size of 1-100 nm.
Likewise, Cowie et al. U.S. Pat. No. 4,927,464 also provides single-species titanium dioxide compositions for absorbing UV radiation. Cowie et al. use acicular titanium dioxide wherein the length of the longest dimension is 10-150 nm and the particles are coated with a mixture of alumina and silica.
Iwaya U.S. Pat. No. 5,032,090 suggests use of a combination of titanium dioxide and zinc oxide in anti-suntan cosmetic compositions to block ultraviolet rays in both the UVB and UVA regions. One drawback of this approach is that titanium dioxide may undesirably whiten or blue the skin in some formulations. Also the use of multiple metal oxides having significant reactivity in many sunscreen systems may complicate the issues a formulator of compositions for human topical application must address. Furthermore, although titanium dioxide is approved by regulatory agencies for many UV-protective applications, it is not approved for use in combination with avobenzone, a popular organic sunscreen agent. In addition, titanium is not naturally present in human and other organisms, and may therefore be an undesired ingredient for some prospective users.
Iwaya U.S. Pat. No. 5,032,090 suggests use of a combination of titanium dioxide and zinc oxide with a primary particle size great than 70 nm to 300 nm in anti-suntan cosmetic compositions to block ultraviolet rays in both the UVB and UVA regions. The primary particle size of the zinc oxide claimed is too large to be effective in UVB.
Other formulations of the prior art use single-species zinc oxide compositions for UV absorption. For example, Kobayashi Kose Co. JP 60-231607 provides anti-suntan cosmetics containing 10-30% zinc oxide having a particle diameter of <100 nm.
Mitchell et al. U.S. Pat. No. 5,587,148 discloses sunscreen products intended to absorb both UVA and UVB radiation while providing a clear appearance on the skin. One embodiment of Mitchell et al.'s disclosure employs micronized particles of zinc oxide having a size up to about 0.2 μm and having what are purportedly reduced levels of toxic heavy metals, which are formulated with a liquid carrier into a colorless emulsion. According to Mitchell et al., this formulation “is capable of absorbing a substantial quantity, if not all, of the UV radiation to which the user is exposed.” (Column 7, lines 15-17.) However, the Mitchell et al. specification provides little, if any, support for this conclusion. Other distinct embodiments proposed by Mitchell et al. include: use of large crystals of zinc oxide, measuring between about 1-100 microns in diameter; and use of transparent plastic spheres measuring between about 0.01-100 microns in diameter which incorporate a UV-absorbing additive.
Cole et al. U.S. Pat. No. 5,340,567 provides sunscreen products intended for UV absorbance. Cole et al. provide mixed metal oxide compositions containing titanium dioxide having a particle size of less than about 35 nm and zinc oxide having a particle size of less than about 50 nm. The metal oxide particles used by Cole et al. are each of a single substantially uniform size.
A disclosure by inventors herein Yun Shao and David Schlossman, “Effect of Particle Size on Performance of Physical Sunscreen formulas” PCIA conference, Shanghai, China R. P. (1999) (“Shao et al. 1999” hereinafter) and available, at the date of this application, describes some of the effects of size, surface treatment, dispersion vehicle, dispersant and other factors on the UV-protective performance of inorganic sunscreens, notably titanium dioxide and zinc oxide. Shao et al. 1999 emphasize the importance of studying size reduction and the relationship between particle size and performance.
Shao et al. 1999 describe use of dispersions, or “pre-dispersions” of titanium dioxide and zinc oxide, intended for formulation with other ingredients to provide useful end product. Shao et al. 1999 concluded at that time that a high loading of solids in the dispersion were important to size reduction and that other factors should also be considered, including pigment selection, surface treatment, vehicle and dispersant. Titanium dioxide is described as providing excellent protection against UVB along with effective UVA protection at a larger size where scattering may contribute significantly. However, such larger sizes may sacrifice some degree of SPF and transparency.
The protection afforded by zinc oxide is considered by Shao et al. 1999 to vary inversely with particle size. Also, zinc oxide is described as providing efficient UVA protection, often with a low SPF. One difficulty these findings present to the worker seeking to provide a broad-spectrum inorganic UV-protective agent suitable for topical application, is that desired sizes of titanium dioxide particles may cause whitening on the skin, as may be understood from FIG. 5 of Shao 1999. (Nor does Shao et al. 1999 describe an adequate zinc oxide formulation.
Another disclosure of inventors, Yun Shao and David Schlossman herein, namely Discovering an Optimum Micropigment for High UV Shielding and Low Skin Whitening, 23rd IFSCC Congress Orlando 2004 (“Shao et al. 2004” hereinafter) describes studies on the UV attenuation of dispersions of titanium dioxide having a primary particle size (“PPS”) as small as 15 nm and of zinc oxide as small as 20 nm which studies include studies of their in vivo SPF efficacy. Shao et al. 2004 conclude that size reduction of titanium dioxide and zinc oxide can remarkably improve the appearance of a sunscreen lotion and can improve the SPF in many cases. However, according to Shao et al. 2004, if the size of the titanium dioxide particles is too small, the energy absorption may shift to UVC wavelengths, weakening the attenuation in the UVA and UVB wavebands. Shao et al. 2004 conclude that zinc oxide could provide an effective SPF at (secondary) particle sizes under 130 nm, but “at the cost of UVA protection”.
As may be understood from Shao et al. 2004, secondary particle size may often be more important for ultraviolet protection than is the primary particle size, but the primary formulator who must provide appealing, functional products which can be provided to consumers in a satisfactory and aesthetic condition.